Module, Fuel Pump: Operation

When the Powertrain Control Module (PCM) senses the ignition status is ON or START it provides an output to energize the Auto Shut Down (ASD) relay and the fuel pump relay in the underhood Power Distribution Center (PDC). The PCM will also ground the fuel pump relay driver at this time, closing the fuel pump relay contacts, completing the fuel pump circuit to chassis ground. If the PCM senses the ignition in the ON position, without a engine crank signal for more than about 30 seconds, it will open the fuel pump driver circuit, shutting down the fuel pump.
The diesel fuel pump module draws fuel through a internal filter and pushes it through the motor to an outlet pipe. The fuel pump module contains two check valves. One valve relieves internal fuel pump pressure and regulates maximum pump output. The second valve, located within the pump outlet, maintains residual pressure during engine OFF conditions. The pressure regulated fuel pump will maintain constant fuel pressure to the fuel injection pump. The fuel pump pressure regulator contains a calibrated spring which forces a diaphragm against the fuel filter return port. When pressure exceeds the calibrated value, the diaphragm retracts, allowing excess fuel pressure in the fuel pump module to purge back into the fuel tank.